1. Field of the Invention
The present invention relates to a sensor and method for measuring the mass flow rate of a gaseous, liquid, or solid material.
2. Description of the Related Art
Sensors for measuring the mass flow rate of flowing, fluidic materials such as air find applications in many areas of technology. Precise control of the ratio of fuel and air supplied into the intake of the engine of an automotive vehicle is possible if the mass flow rates of the individual constituents can be accurately sensed or measured. Mass flow sensors which have been used for this purpose in the past generally require that a mechanical element be disposed in the fluid flow path. The mechanical element may be displaced, rotated, etc. by an amount corresponding to the fluid flow rate.
Alternatively, the obstruction may be in the form of a fixed member such as a Karman vortex shedder as disclosed in U.S. Pat. No. 4,815,324, entitled "INTAKE AIR METER FOR AN INTERNAL COMBUSTION ENGINE", issued Mar. 28, 1989, to Y. Tada et al. The shedder generates a vortex street which phase modulates an ultrasonic signal. The modulation frequency increases with the flow rate of the fluid. The phase modulated signal is received and processed to compute the flow rate of the fluid.
Generally, any flow rate sensor which requires an obstruction is undesirable since it creates turbulent flow and resulting inefficiency in applications where laminar flow is preferred.
A flow rate sensor which utilizes two contrapropagating ultrasonic waves which are periodically switched back and forth for measuring fluid velocity, rather than true mass flow, is disclosed in U.S. Pat. No. 4,320,666, entitled "FLUID FLOW MEASURING APPARATUS", issued Mar. 23, 1982, to R. Redding. This reference teaches the use of two phase locked loops for separately measuring the propagation times of the contrapropagating signals on a time sharing basis. Although eliminating the obstructions required in the Karman vortex systems, switching between the two signals limits Redding's system to applications in which relatively slow response times are tolerable. Automotive applications require a 15 millisecond response time, which is unattainable in an ultrasonic system such as Redding's with periodically switched signals.
In addition, automotive applications require measurement of true mass flow rate, rather than material velocity. The accuracy of a system such as Redding's which directly measures only velocity, and which may calculate a mass flow rate using approximated, rather than measured values, is insufficient for this purpose.
My previous U.S. Pat. No. 4,829,305, entitled "MEDIUM DISTANCE MEASUREMENT SYSTEM AND METHOD", issued May 9, 1989, discloses a system including an optical signal and a phase locked loop arrangement which produces an output signal having a frequency which varies as a predetermined function of the distance from the sensor to a target. This is accomplished by adjusting the frequency of the optical signal which is transmitted to a target in such a manner as to maintain a constant phase difference between the transmitted signal and an echo of the signal reflected back to the sensor from the target. The principle of frequency adjustment to maintain a fixed phase difference between two signals as disclosed in my prior patent may be advantageously incorporated into a mass flow rate sensor embodying the present invention as will be described in detail below.